Signal delay device



Jan. 27, 1953 w, HOLMAN 2,626,992

' SIGNAL DELAY DEVICE Filed Feb. 26. 1949 s Sheets-Sheet 1 E. W HOLMAN ATTORNEY Jan. 27, 1953 w HOLMAN I 2,626,992

SIGNAL DELAY DEVICE Filed Feb. 26. 1949 .3 Sheets- Sheet 2 ATTORNEY Jan. 27, 1953 E. w. HOLMAN SIGNAL DELAY DEVICE 3 Sheets-Sheet 3 Filed Feb. 26. 1949 IN l E N TOR m H01. MAN

A T TORNEV "a :del'aytime oi -about 5,000 microseconds. "problem that presents itself inthe case of such -alongf1ine is*the transmission characteristic of Patented Jan. 27, 1953 SIGNAL DELAY DEVICE Erwin W. Holman, Summit, N. J., assignor to=Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 26, 1949,-Serial' No. 78,503

10 Claims. 1

The present invention" relates to theconstruc- 'tion and use of a transmission delay line comprising a fluid-filled pipe for transmitting mechanical impulses from :one end of the line to the oppositeend in order to-provide a particular stime..-interval of transmission for the pulses.

Such a :delay line isuseful in many fields of application, such as in signaling systems in which an .electrical'im'pulse that: is to be delayed is converted'to a mechanical impulse at one end of the 'lline-and the delayed mechanical impulse at the other end of .the: line is again transformedinto an electrical impulse thus providing a longer delay tn-:the electrical impulse than is feasible by-purely electrical circuit means.

The .present invention, in common with the applicatio'nof Herbert J. 'Mcskimin, Serial 'No.

-653 255, filed March 9, 1946, now Patent No. 2,505,364, issued April 25,1950, provides a'relatively long fiuidtransmission line Which for compactness may be-folded, with reflectingpoints at Jfolds. The construction in the present applica- -tion is in general similarto that disclosedin the Mcskimin application, and the theory of propagation of waves and the theoretical analysis and discussion giventherein and in the article by H. Mcskimin entitled Theoretical Analysis of the' Mercury Delay Line published inthe Journal ofithe Acoustical'society of America, July 1948, are-generally applicable to the transmission delay line to be specifically disclosed in this application.

The line "tobe: specifically disclosed herein by waypfiexample ls' a mercuryline twenty-four feet long in contrast to-prior art line of the order of two to fourfeetlong. A line of this length has One the line itself since its attenuation and rate of in- "crease of-attenuation with increase of frequency are large enough to become significant in deter- -mining the overall transmission characteristic -o'fline -pl=us-the terminal transducers, such as piezoelectric crystals.

Another problem that becomes accentuated in -the case of such a long line lies in'the mechan- .-ical difficulty of centering sufficiently accurately thebeam upon thetransducer atthe far end of theline.

'The invention comprises among its features such a combinationof transducer.andtransmission line characteristics as will resultin the de- -sired-overall transmission characteristic, as to both attenuation and band width. A further feature is a typeof .transducermounting that will transducer element after theline has beencompletel-y assembled and filled with mercury or. other fluid-so as .to enable the. best transmission characteristic to.be; obtained.

A furtherfeaturecomprisesprovision for main. taining intimate mechanicalcontact-between the mercury or-otherfiuidandthe entire crystal face under all conditions ofvoperation.

Further features have to. do with the constructional details of the "apparatus, particularly of the mountings for. thedriving and driven crystals.

While there aremany types of liquid filling that canbe used inidelaylines forthe propagation of the waves and, the improvements according tothis invention are notconfinedto the useofa particularfilling, .thedescriptionirom this point on will-be.directed to. a line-havingmercury as-the medium and with the assumption that a considerable frequency band width is tobe transmitted. It will also be assumed in thedescription thatpulses are to be sentthrough the-delay line and thatthe formofpulses is to-be faithfullypreserved, requiring the transmission of a .sufiiciently. broad .frequency band. Actually, the

pulse is transmitted as..a.m0.du1ation'of.a high frequency .carrier Wave which may be i of the order, for examplepof. five to ,ten, megacycles.

The invention .may be better understood by reference to the drawings in which:

Fig. .1 is a. schematic drawing showing the arrangement ofthe folded delay line;

Fig. 2 is aperspective view ofone of the crystal head units, .detached from the assembly, and

.shown partially broken away .and in section to show the details of construction. more clearly;

Fig-3 is. a perspective .view showing a. complete folded. delay-line. unit Fig. .4 is anend view of one of the assembled crystal head-units looking in the direction indicatedby lined-.4 of Fig. 2;

Fig. 5 is-a detailview of a portion of themeans for aligning the crystal, taken as .indicated by line. 55,.0f, FigA;

Fig. 61s a fragmentary view of the end assembly taken asnindicated-by line-66 of Fig. 3, and partially brokenaway to show details-of the seals between thehead unit andthe tube;

Fig. 7 isa-top-view, takenas indicated by line 1-1. of Fig.3, andpartially broken away to show details oflthe-endzreflectors and Sylphon'bellows an Fig.18 shows graphs of transmission character- 156105130 be: referred to--in the description.

'filled tube l2.

It is to be understood that the embodiments shown are illustrative only of the invention and that other equivalent structure may be used and other materials used Within the invention.

Quartz crystal discs (X-cut) are used at both ends of the line for converting between electrical and mechanical energy. These are half a wavelength thick at the fundamental resonant frequency and the surfaces of the crystals in contact with the mercury are highly polished and must be as perfectly clean as possible. No attempt is made to back up these crystals with a mechanical impedance matching that of the crystal. In fact total reflection from the back of the crystal through the crystal and into the mercury reinforces the original waves in phase and gives a six-decibel gain. The attenuation of the line is so high (about 20 decibels) at the operating carrier frequency of 6.5 megacycles that standing .waves are maintained sufficiently low not to be troublesome. the reflection from the back of the crystal being 40 decibels down with reference to the transmitted waves.

The reflecting surfaces at the fold points of the line are heavy steel plates. The surfaces of these reflecting plates and the inner surface of the tubes are roughened as in the McSkimin application disclosure to promote reflection by accentuating the impedance mismatch.

Referrin for a detailed description to the dra in s. Fig. 1 shows schematically the layout of the folded line assembly I. An electrical impulse to be dela ed is applied at the transmitting cr stal head 4 where it is converted into mechanical vibrations. These vibrations are propagated throu h mercury-filled tub-e 5 to end refiectors 6 and I. each of hich deflect at 90 degrees. They then travel throu h mercury-filled tube 9 to a second pair of end reflectors l and II which produces a further 180-degree reflect on sendin the vibrations throu h mercury- A third set of reflectors l4 and I directs the vibrations back throu h mercuryfilled tube IE to the receiving crystal head assembly H. The mechanical im ulse there is converted back into varying electrical potentials and a lied to the output circuit in form identical with those ori inally sup lied but delayed by the length of time reouired for the mechani-- cal vibrations to pass through the mercury column The circuits associated with the crystals are schematicallv indicated. The carrier wave source S which may supply a pulsed carrier wave. by use of means symbolically shown as a key K. is connected bet een the crystal 4| of terminal 4 and tube 5. The crystal has an inherent capacity 0, shown in dotted outline. which is resonated with an inductance L at the carrier frequency and the resonant circuit is provided with a damping resistance 1' to give the desired band width. The receiver R. is shown connected by a similar intermediate circuit to the crystal 4| and line I6 in receiving terminal H.

In order to insure that the column of mercury fills the tubes completely or that there is otherwise adequate contact with the crystal faces, an auxiliary supply of mercury is contained in Sylphon bellows l9 and 20 which are kept under pressure by suitably anchored springs 2| and 22. The importance of maintaining proper contact between the crystal faces and the mercury and between the mercury and the walls of the tubes will be discussed in greater detail hereafter. For the present it is sufiicient to say that proper contact is secured by such connection of auxiliary supplies of mercury to the tubes through conduits 24 and 25, regardless of pressure or temperature variations, and of inclination of instrument at either end. I

The construction of the crystal head assembly which permits the extremely precise adjustment and alignment of the crystals necessary to the proper use of such a line, is shown in Fig. 2. Castellated screw fitting 30 is provided for the reception of a coaxial line terminal or other connecting line. This connection element includes a mounting plate 3|, and a plug 32 which is an extension of the central conductor of the coaxial line. Plug 32 is designed to fit slidably into a crystal backing member 34, and to provide good connection thereto while permitting angular movements during alignment of the crystal. The mounting plate 3| is secured fixedly to a screw plug 35 so that the entire crystal and its adjusting support members may be removed as a unit from the housing 36 which is secured to the line assembly I, in such fashion as to be accurately adjustable relativel thereto, by means to be described in detail hereafter. The crystal backing member 34 comprises a hemispherical metal support 31 from the rear of which project slotted resilient contact fingers 39, which directly engage with the plug 32. Opposite fingers 39, the hemispherical support has a planar face 43 with which the flat circular crystal 4| is held in contact by a flange 42, projecting inwardly from an annular member 44. Member 44 surrounds a hard rubber ring 45, which presses against the hemispherical portion of the crystal support 31 and is sealed thereto by means of a washer 43, over which is crimped a thin edge portion 41, of the annular member 44. The assembly inclosed by the annular member 44 is sealed by means of an annular gasket 49 against the housing 36 under a pressure against the back of rubber ring 45 produced and controlled by the screw plug member 35. Due to the hemispherical shape of the portion 31 of the backing member 34, the conical shape of the rubber ring 45 which fits thereover, and the motion of the member 34 permitted by the resilient fingers 39, if the face 4|) tends to exert non-uniform pressure on the crystal 4| the member 34 will reseat itself in such a way as to exert uniform pressure upon the back of the crystal 4| over the area of contact. This feature prevents cracking the crystal 4| as the plug 35 is screwed into place. The housing 36 is in turn sealed to the tube assembly by means of a flexible gasket 50, compressed between housing 36 and base plate 5|, by screws 5|. Base plate 5| is secured to end plate 52 by means providing the precise alignment which are shown in Figs. 4 to 6 and will be described in detail hereafter.

Fig. 3 shows the construction of the assembly shown schematically in Fig. 1 in greater detail. Two channel beams 68 and SI are used to provide the necessary sup-port for the tube assembly. These channel beams are suitably secured at their ends and are provided with transverse members 52 and 64', in order to support the tubes rigidly. It should be recognized that the mass of the mercury filled pipes is considerable and that when a pipe of 6 feet in length is used, it is very necessary to provide support intermediate its ends or else there will be suiiicient sag to cause spurious impulses to be observed at their receiving end. The tubes 5, 9, I2, l6 are in the preferred embodiment of my invention constructed as stainless steel .zegccegcaa mipesrl'iaving amtintemal :diametercof: onezinch. rEor :stationary user-suchttubes..may:be in ssome zcasestsufiiciently'selfesupportingzrbut'zwhere itriS ccontemplatedzthat they. will. beesubiected to rough -ihandling provision must be made forsuitable sup- :port. '.tThe rtnansmittingzandrreceiving crystal'end cassembiiesckand ll'aare shown in the' left'iforeground of Fig. 3. At the oppos-ite'end are scenthe .rreflectors '1 hand Hi, while vidirectly above 'the acryst'alaheads areseen the reflectors It) hand I I. =.,10pposite therheadsassembly is. seenthe. housing i=65i-in'which is included a-Sylphon bellowsZU, providedrorethemaintenanceof the mercury supply.

connection-with reflectors ill-and l La bleed- .jngaperture 66,;:closed; :bya screw plugcfi'l, is pro- Visdedvin order. to -permit the release. of .nccluded .azgases. Bleeders' are-likewise; provided ;in the. fit- ==tingsadjacent reflectors 1. and. M andthesylphon ibellowsl l'9ta-ndi20. Aperture'BB is also usedfor filling. the line with mercury.

The means foradjustingandaligning the crystal head units precisely will next be considered in connection with Figs. 3 to 6 inclusive. The base .plate 5!. hasibeendescribedas secured to the end .1. plate 52 by screws (see Fig--65) This isaccom- .plished b-y means-.of-screws it whiehare con- \yeniently provided with socketed heads H. The .basepla-te 5| is clamped against end .plate 52 withuan. annular? gasket 'of resilient material 12. lhis material must be'of the type which not afiected by contact .with'mercury and afi'ords' ..a-sealabout-the junction between the crystal head auniband the first; piped -A- slight space is. left between and plate 52 and base plate -5 l which perl mitst-ther latter tobe-adjusted relative to the.

-former about=tha gasket asa pivot by means -of tthe adjusting screws 10. This-adjustment may needto be. as slight as of..the order of ..00001 of anv inch .and. it. is'importantthat, once made, it

shall not. change. .In order'to preventvibration from loosening the screw 10,. two adjusting bolts 14 are mounted adjacent the socketeclhead H, and are usedto hold a cover p1ate'15, against the socketed head I l. Conventional lock washer lfimay be used to prevent the adjusting bolts! from changing their position.

that other equivalentmeans'may be used to secure "thepositioning. bolt Hiin place. .It shouldbe inote'dzhoweventhat when lock washers have been 'LllSd 'withfthis bolt "directly, strains have been set up 'inlthe base'jplate such as 'to throw'the "crystal heads outiof alignment. Hence I find. it desirable tojprovide some method of maintaining the adjustment of the positioning bolts which willnotiintroduceiwarping or distortion in the end j'platepr base; plate'and this can be done by the introduction or a'suitable" lock either rigid orfric- .tional .which.may.be readily removed topermit readjustment of the crystal head. In Fig. 6 is seen e'rlsothejoint between the tube 5i and'the end.

Therletails of theSylphon bellowsarrangement,

.2201 for? maintainingaa constant supply of mercury narenshownwinPigfl. In this figure-a top view-is e-shown partially broken away to. permit easierinspection. at the details of construction. vTube 3 receive 'vibrations from the tubes! therebeneatm.

It i is recognized aofssteel. removably by means of :socket 'head bolts 8|.

that the yerticalwchannelzbore '80;may1be seen "closed: at; its; lower endiby the "reflector 6.

The :construction .of these end: reflectors is preferably .The end reflectors 5 6 and :1 r are held They may be taken :off iorinspectiontor cleaning of the interior. It should :benoted that the bore consists of threeseparate; passageways machineddn the solid :blockz88. A bore must the providedinscontinuation of each of the pipes 5 .a and 9.together:with a boreat'right. angles toxboth of them. .The passage f thus formed communicates with the Sylphon .bellows 20 :through a Ti-shaped. fitting 82 provided .with ableeder plug :84. -The bellows assembly is enclosed in a housing65 'and includes a spring 85 plus a stub shaft .-36,which acts to-maintain the axial alignment In. assembling .the line; the tubes andthe end assemblies -at-'the-.refiectingpoints are put together in themannerxgenerally indicated in the drawings but without any fluid filling initially. The-reflecting :plates at the folds, that is, 6, '7; 10,:ll-and 14, 15' are left off and in their places .aredbolted a corresponding number of optically flat mirrors.

The crystal mounting assemblies including housingsJiB-andyplates 5I are likewise left on. and smalloptical apertures are provided win-their stead: at theend'of the pipes 5 and i6.

Light is projected through one of these apertures and the image of the aperture is observed at the opposite: terminal of the line. Adjustments are .made to the extent necessary to-secure precise centering of the image; by placing shims between the head block and the block 88 (Fig. '7) supporting the mirrors. By this procedure accurate axialalignment of the entire line is achieved.

If necessary. the. alignment. can be accomplished .in.steps.by usingtheoptical test on two lengths of theli-ne atfirst, ,thenon three lengths and soon.

rAfterthe opticaltest has been. completed and the; adjustments made in connectioniwith it, the mirrors are, replaced by the reflectingsteel plates and the crystal mountingheads are secured to the two ends of the-line. .Mercury is .then poured into port 66,-with the bellows-.65 held at its mid- .position by substituting a block forthe spring85.

.By tilting the lineslowly back and forth the. air

scan .be. madeto accumulate atthe end of the line where-theport =66 is located andcan thus be removed. :Alternatively, the entire line can first and thereafter. the mercuryv canbe distilled into vantagestwhich-show up in connection with the .usepf theline .such, as making it more immune to mechanical shock and vibrations in the line, which when present have the effect of increasing the attenuation of the line for the transmission of the desired waves. However, to assist in achieving proper reflection at the reflector plates, it is desirable to introduce a microscopically thin layer of air, which may be held on the surface of the plate merely by occlusion. When the line is filled under vacuum, one way to provide such air films on the faces of the reflector plates is to stand the line on end after it has been filled, allow the bellows 65 to expand somewhat and remove the reflector plates at the end of the line that is uppermost. This permits air to enter. The plates are replaced when the level of the mercury is brought close to the plane that is to be occupied by the reflecting plates and the plates are then bolted in place. Lap grinding can be used on the face of each plate adjacent the mercury to promote formation of an air film. Lap grinding is also of advantage on the face of the quency bands to be used since crystals have broader resonance bands at high frequencies than at low frequencies. Although the summation curve S of Fig. 8 is sloped and has curvature, these effects can be largely compensated by use of suitable equalizers in the external circuits connected to the crystals. Two such equalizers are designated EQ in Fig. 1.

The frequency f0 is the resonant frequency of th crystal. Frequency J is what may be termed optimum operating frequency since it represents the point of minimum over-all attenuation as given by curve S. Operation at frequency ;i may give such low attenuation that the standing waves become troublesome in which case it may be advantageous to operate at some higher frequency such as at o. This is generally at the sacrifice of some band width since the slope across the band is not symmetrical on both sides of f0 and is therefor difficult to equalize. Greater band width without equalization is available by operatbacking plate 3t for the crystal 4! to increase the mismatch of impedances and promote reflection. If it is not desired that the line be opened and exposed to the air, a sheet or film of paper or other low impedance material may be used to cover the steel plate.

With the line finally assembled and filled, the spring is restored in the bellows chamber so as to hold the mercury in good physical contact with all inner surfaces of the line including crystal faces and reflector plates.

Test pulses of the high frequency carrier wave are now sent through the line and by means of an oscilloscope and known type of testing circuit a comparison is made between the shape of the pulse when transmitted through the mercury line and the shape of the pulse when transmitted through a distortionless line which may conveniently be in th form of a calibrated attenuating circuit. It is quite likely that the pulse form at first observed will not be the best obtainable. This is because of the cliiference in conditions obtaining when the line was empty, as it was when the optical tests were made, and when it has been filled with the fluid especially in the case of mercury which is very heavy. This is where the adjustable feature of the crystal heads accordin to the invention is of great advantage for it is only necessary to adjust the bolts ill to produce micrometer shifts in orientation of the crystal heads, the yielding washer l2 allowing such movements to be made without disturbing the rest of the line or introducing strains into the crystals or their mountings or into any other part of the line. The procedure is to make slight adjustments in the crystal heads while observing the pulse shape on the oscilloscope. The adjustments are continued until the received pulse shape is the most perfect replica of the transmitted pulse obtainable.

The shape of the received pulse depends markedly upon the over-all band width of the line and crystal. The band width at relatively low frequency is primarily determined by the crystal resonance but since the line attenuation increases approximately as the square of the frequency, it becomes controlling at high frequencies from the standpoint of band width distortion. The line and crystal characteristics for one particular line are given in Fig. 8 by the curves M and N respectively, the overall characteristic being given by the summation curve S. Operating the line at high frequencies means acceptance of increased attenuation or line loss but allows broader freing at frequency ,f.

What is claimed is:

1. In a signaling system, apparatus for reproducing in an output circuit a signal pulse which is a replica of an input circuit signal pulse and delayed with respect thereto which comprises an angularly folded conduit, a reflector at each angular fold of such conduit, a piezoelectric crystal driver element at the head of said conduit, a piezoelectric crystal receiver element at the other end of said conduit, a low-loss high impedance liquid substantially filling said conduit, both faces of said crystals adjacent said liquid being highly polished to provide intimate contact therewith, a backing plate adjacent the rear face of each of said crystals, the texture of the surface of said backing plate adjacent said crystal being adapted to increase the mismatch of impedances and promote reflection of compressional waves, and means for maintaining constancy of hydrostatic pressure of said liquid for all positions of said conduit.

2. In a delay transmission line comprising a fluid-filled tube of relatively great length, a piezoelectric crystal at an end of said tube for converting impressed electrical pulses into mechanical pulses in said fluid, and a backing plate for said crystal, said crystal having a sufficiently high resonant frequency to possess the band width necessary to transmit the pulses true to form, said frequency being sufliciently high that standing waves in said line are not troublesome, said crystal having a thickness of halfa wavelength at the resonant frequency, and the side of said backing plate facing said crystal having good reflecting properties.

3. A system according to claim 2 in which the impressed electrical pulses are pulses of high frequency carrier waves, the carrier frequency coinciding with the resonant frequency of said crystal.

4. A system according to claim 2 in which the optimum operating frequency is different from the resonant frequency of said crystal and in which the impressed electrical pulses are in. the form of pulses of high frequency carrier waves, the carrier frequency being substantially at the optimum operating frequency of the system.

5. Apparatus in accordance with claim 1 which includes means for equalizing the pressure between each of said crystals and its associated backing plate over the area of said crystal.

6. Apparatus in accordance with claim 1 in which said liquid is mercury which contains substantially no occluded air.

7. A delay line in accordance with claim 2 which includes means for maintaining within said tube a hydrostatic pressure greater than atmospheric pressure for all positions of said line.

8. A delay line in accordance with claim 2 which includes means for equalizing the pressure between said crystal and said plate over the area of said crystal.

9. In a delay line of the type comprising a folded tube filled with liquid, piezoelectric crystals at the respective ends thereof, and an acoustically reflecting plate at the fold of the tube, the method of axially aligning the delay line which comprises removing the liquid, substituting for the crystals small optical apertures centered at the respective ends of the line, substituting an optically flat mirror for the plate, projecting a beam of light through one of the apertures, and adjusting the plane of the mirror until the beam emerges from the other aperture.

10. In combination, a container, liquid filling said container, an aperture in said container, a piezoelectric crystal closing said aperture from the outside, a backing plate in contact with said crystal over substantially the entire outer face 10 thereof, a resilient member contacting said plate, and adjustable means for exerting pressure upon said member to force it against said crystal, the back of said backing plate being hemispherical in shape, and said member being conical in form on the side facing said backing plate.

ERWIN W. H-OLMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

